Patent GB-652291-A (GB)
Title: Improvements in and relating to electrical protective systems
IPC: H02H3/30
Filed: 1948-02-24
Abstract: 652,291. Protective cut-out arrangements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Feb. 24, 1948, No. 5479. Convention date, May 25, 1945. [Class 38(v)]  In a protective arrangement of the phase comparison type for a polyphase feeder section, a phase sequence segregating network is arranged at each end of the protected section so as to derive under unbalanced fault conditions a single phase voltage dependent on the negative sequence component only, or on the negative sequence component with the addition of a zero sequence component; whilst under balanced fault conditions the derived voltage is modified by, or replaced by, the positive sequence component. Under fault conditions, each network applies an operating potential to an associated comparison relay during alternate half cycles and during the other half cycles, operates a high frequency transmitter to send a blocking signal to the other end of the section, the arrangement being such that under through fault conditions each comparison relay receives a blocking signal during the appropriate half cycles to prevent its operation. Fig. 1 shows one arrangement for one end of the protected feeder section. Unbalanced fault conditions. - Current transformers 9.. 11 supply the primary windings 17, 18 of transformers 15, 14 having secondary windings 24, 21 interconnected by resistors 19, 20, 23 and capacitor 22, arranged so that the voltage across the output circuit 13 of this network is dependent on the negative sequence component, the zero component being removed by the effect of additional primary windings 25, 26 on the transformers 14, 15. These windings 25, 26 are connected so as to be energized by the sum of the zero components and normally have one-third of the effective turns that the windings 18, 17 have, so that the voltage induced by them in the secondary windings 21. 24, cancels out that induced into the secondary windings by the zero components in windings 18, 17. However, the effective turns of winding 25 may be adjusted so as to introduce the zero component if desired.  Balanced fault conditions.-Part of the resistor 20 is normally short circuited by the parallel-connected contacts 31..33 of relays 28 : 30, which have their operating coils connected in series with the current transformers 9 ... 11. When balanced fault conditions occur, all three relays open their contacts and remove the short circuit from the resistor, causing the voltage drop across it to increase. Thus the voltage drop across the resistor 19 no longer equals the opposing voltage across the capacitor 22, and therefore a voltage is obtained across the output circuit 13 which is dependent on the positive sequence component. The short circuit would not be removed from the resistor 20 under unbalanced fault conditions as not all the relays 28-30 would open. Operation under through fault conditions.-The single phase voltage across the output circuit 13 is applied to a comparison relay C, when the voltage is of sufficient magnitude to close a relay 40, this value being determined by that necessary to operate a high frequency transmitter T, which is also connected across the circuit 13 and transmits over one of the power lines. The comparison relay C is effective to close its contacts 38 to energize a circuitbreaker trip coil 6 when it does not receive a blocking voltage transmitted from the other end of the feeder section. The comparison relay C, transmitter T and receiver R, shown diagrammatically, are arranged so that the relay C is energized by the output circuit 13 during alternate half cycles. whilst the transmitter T transmits during the other alternate half cycles to the receiver at the other end. The networks at each end are also arranged so that under through fault conditions. the polarities of the output voltages are 180 degrees out of phase, so that as the comparison relays C at each end are energized by their associated circuit 13, they are blocked by means of the transmission from the other end. Operation under internal fault conditions.-When currents flow into the section at each end due to an internal fault, the polarities of the output voltages become the same, so that the high frequency transmissions from each end occur during the same half cycles. which are not the half cycles in which the relays C are energized. Hence the relays C close their contacts 38 to energize the circuit-breaker trip coils 6. In a modification (Fig. 2, not shown), the relays 28.30 are arranged to change the network connections, so that under balanced fault conditions the network is changed from a negative sequence arrangement to a positive sequence arrangement.